Physicians and health experts generally agree that a diet low in saturated fats and cholesterol can reduce the likelihood of heart and circulatory diseases. Consumer awareness of the health benefits associated with maintaining a diet low in fat and cholesterol has recently increased, along with the demand for food products low in these components. Because of its low fat content, skim milk currently has large applications in such lowfat food products.
Whole milk is a dilute emulsion combined with a colloidal dispersion in which the continuous phase is a solution. Whole milk has a fat content typically between about 3.3% to about 3.4% and 14 mg cholesterol per 100 g milk. To obtain skim milk, whole milk is centrifuged at a low speed. An oil rich phase having cream floating on top, and a liquid phase or skim milk is obtained.
In milk products, the majority of the fat and about 80 percent to about 85 percent of the total cholesterol is present in the cream. The cream is comprised of predominantly milk fat globules. The cholesterol in the milk fat is thought to be distributed between the milk fat globule membrane and the bulk lipid. Wong, Fundamentals of Dairy Chemistry (1988). When the membrane is separated from the milk fat globules and the butter oil is isolated from the milk fat globules, about 90 percent or greater of the cholesterol is equilibrated in the butter oil and about 5 percent or greater is in the membrane.
Roughly, the skim milk contains about 10% to about 20% of the cholesterol that is in whole milk. Skim milk contains less than about 0.5% fat, about 10% solids and typically about 2 to about 3 mg of cholesterol per 100 g of skim milk.
The cholesterol in milk products is thought to be associated with triglycerides, milk fat globules and complex proteins. Cholesterol in skim milk is thought to exist in three forms: (i) complexed with residual triglyceride droplets not removed in the skimming process, (ii) complexed with lipoprotein particles sloughed off from milk fat globule membranes in the skimming process, and (iii) complexed with proteins contained in the serum albumin. When skim milk, reduced-fat or whole milk is concentrated, its cholesterol content increases proportionally. For example, nonfat dry milk has a cholesterol content of about 20-30 mg per 100 grams. Therefore, the use of skim milk, reduced-fat or whole milk as an ingredient in low fat foods can contribute significant amounts of cholesterol to these foods.
It is therefore desirable to produce milk products that have a substantially reduced cholesterol content. A satisfactory cholesterol removal process would maximize cholesterol removal without affecting the protein functionality or other properties of the milk. A desirable removal process would be simple to perform and would minimize equipment and raw material requirements. Furthermore, the use of potentially harmful materials such as organic polar solvents would preferably be avoided. No such method is known to have been developed prior to the present invention.
Several approaches have been utilized for removing the cholesterol from milk fats. U.S. Pat. No. 2,613,215 and EP No. 174,848 describe methods of removing cholesterol from fats by contacting with adsorbent materials such as silica gel and activated carbon. When applied to milk products, such adsorbents have been found to either be too impractical for commercial use or to lack specificity for cholesterol adsorption.
The use of supercritical extraction to extract cholesterol-containing triglycerides from butterfat is described in U.S. Pat. No. 4,504,503. Supercritical extraction, however, involves extreme process conditions and is generally too expensive for large commercial applications. Therefore, a more economical process is desired.
U.S. Pat. No. 4,997,668 discloses a process for the reduction of cholesterol in separated milk fat by solvent extraction without significantly altering the character of the milk fat globules. This method requires an organic polar solvent which must be removed by vacuum filtration to avoid residual solvent in the product.
Edible vegetable oils have been used to extract cholesterol from egg yolks, as described in U.S. Pat. Nos. 3,617,474; 4,103,040 and 4,333,959. However, the amount of oil typically required to satisfactorily extract cholesterol from egg yolk is 2 to 5 times the weight of egg yolk. Accordingly, the use of this extraction is economically disadvantageous in that a tremendous amount of oil is required, which thereby results in a large waste stream of spent oil.
The following other references are cited as background information: U.S. Pat. Nos. 4,255,455; 4,882,193; 3,563,765; 4,296,134; 4,971,827; 4,804,555; 4,234,619; 3,881,034 and 3,594,183.